1. Field of the Invention
The present invention relates to an aqueous pigment ink suitable for ink-jet recording methods.
2. Description of the Related Art
Among the various current recording methods, ink-jet recording methods are nearly noiseless during recording and are capable of recording at high speed, and are particularly noteworthy because recording is accomplished on plain paper without a special fixing process.
In general, aqueous dye inks comprising a water-soluble dye dissolved in an aqueous medium are used as ink-jet recording inks. When such aqueous dye inks are used in ink-jet recording, the recorded image is influenced by the physical properties of the aqueous dye ink and may disadvantageously show deterioration in respect to moisture resistance and light fastness. In the case of black inks in particular, a plurality of dyes are mixed for use as black ink because there are few water soluble dyes which have excellent black color reproducibility; however, a disadvantage arises insofar as black color tone changes when some dyes fade or discolor. This type of ink containing aqueous dye does not have high storage stability.
In recent years, aqueous inks using pigments such as carbon black and the like have been experimentally adapted to ink-jet recording methods in place of the aforesaid inks using dyes. Images formed by the ink including carbon black has excellent moisture resistance compared to that formed by the ink including water-soluble dyes because it is insoluble in water, and further provides excellent light fastness.
A disadvantage arises, however, insofar as the insolubility of carbon black in water, i.e., the ink medium, makes it difficult to assure the stability of carbon black finely dispersed in an aqueous medium. Further disadvantages arise when dispersion agent such as water-soluble resin or the like is added to improve the dispersion stability of the carbon black due unstable continuous discharge resulting from clogging of the discharge nozzle due to the viscosity of the medium.
In order to eliminate the aforesaid disadvantages, U.S. Pat. No. 5,609,671 discloses an ink having excellent dispersibility in water without adding dispersion agent such as water-soluble resin or the like by using a carbon black having a specific amount of surface active hydrogen.
A further disadvantage arises, however, in this aqueous ink containing a dispersion of carbon black provides inadequate glossiness of obtained images. This disadvantage produces a sense of unease in images as the black color fades when color images are reproduced with color inks. Further disadvantages arise such as the detachment of the carbon black from the surface of the print sheet when the image is immersed in water. This detachment is thought to be due to the low bonding strength of the carbon black to the print sheet compared to dyes, because the carbon black particles are simply bonded to the surface of the print sheet whereas dyes bond to the print sheet on the molecular level.
New disadvantages arise when the aforesaid aqueous ink containing a dispersion of carbon black is used for printing in an ink-jet printer, insofar as fluctuation environment temperature produces variation in the quality of the printed image.